The US has removed several more officials from China over fears they have contracted the same mysterious illness that affected staff in Cuba.
The employees, who were working in the southern city of Guangzhou, had reported hearing odd noises.
Last year, 24 US staff working at the Cuba embassy suffered brain injuries after reporting "auditory sensations".
The incidents have raised concerns that a government or agency may be targeting the US with a new type of sonic weapon.
The cases come at a time when China-US relations have been strained amid fears of a trade war.
Earlier this month, the State Department issued a health warning to its staff saying an employee in China had reported "subtle and vague, but abnormal, sensations of sound and pressure".
It said it was taking the reports seriously, but did not yet know the cause, and warned staff to move to a safe place if they encountered any "unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises".
One US official was diagnosed with mild brain trauma, the same injury that affected the Cuban embassy staff.
The State Department has warned that US diplomats should alert their mission's medical staff "if they note new onset of symptoms that may have begun in association with experiencing unidentified auditory sensations".
The department said it had sent a team to Guangzhou and set up a task force to oversee the response to the mystery attacks in China and Cuba.
Cuba has denied targeting embassy staff, and the US has not blamed the country's government for the suspected attacks.
Symptoms of a sonic attack may include dizziness, headaches, vomiting, bowel spasms, vertigo, permanent hearing loss and even brain damage.
"US medical professionals will continue to conduct full evaluations to determine the cause of the reported symptoms and whether the findings are consistent with those noted in previously affected government personnel or possibly completely unrelated," State Department spokeswoman Heather Nauert said in a statement.